This invention relates to an exhaust gas cleaning apparatus for cleaning exhaust gas from a Diesel engine, more specifically to an exhaust gas cleaning apparatus which cleans exhaust gas from a Diesel engine by catching particulates mainly composed of carbon contained in the exhaust gas.
Exhaust gas from a Diesel engine contains particulates mainly composed of carbon. In order to clean such exhaust gas, the particulates must be removed from the exhaust gas. Conventionally, these particulates are caught and removed by using a filter formed of a ceramic of honeycomb structure or a foam structure in an exhaust system. However, if this filter is set in the exhaust system so that the exhaust gas is passed through the filter, the passage resistance of an exhaust passage will be increased by the particulates caught by the filter to lower the engine output.
As measures to counter these drawbacks, there are proposed cleaning apparatus of three types. In a cleaning apparatus of a first type, the caught particulates are burned by a burner. An apparatus of a second type is set near the exhaust port of the engine so that the particulates are burned by operating the engine under such conditions that exhaust gas from the engine is at a high temperature. In a third type, the particulates are burned by electric heat from an electric heater. The cleaning apparatuses of these types, however, are subject to the following drawbacks.
(a) Burner-type cleaning apparatus:
Requiring a fuel supply system and an ignition system, this cleaning apparatus is inevitably complicated and expensive. Moreover, fuel led into the exhaust system will increase the danger of a fire.
(b) Cleaning apparatus set near engine:
Subject to great vibration and high temperature, this apparatus requires additional mechanical strength. In order to raise the exhaust gas to a high temperature (600.degree. C. to 700.degree. C. or more) required for cleaning and regeneration during normal operation, it is necessary that special operating conditions such as air-fuel ratio be improved and that the injection timing be changed. Thus, the output is lowered, and specific fuel consumption is deteriorated.
(c) Electric heater-type cleaning apparatus:
Having an electric heater integrally built-in, this cleaning apparatus must always heat the particulates to a temperature necessary for regeneration. Accordingly, this apparatus requires a great deal of electric power, exerting a bad influence upon the engine and car system. If a conventional metal wire heater is used as the electric heater, it will readily be corroded by exhaust gas to make the cleaning apparatus limited in durability.